Android deep link is a URL that takes users to a specific page of a mobile app, enabling seamless user experience from a webpage to an app. Android app link, a new type of deep link introduced in Android 6.0, is claimed to offer more benefits, such as supporting instant apps and providing more secure verification to protect against hijacking attacks that previous deep links can not. However, we find that the app link is not as secure as claimed, because the verification process can be bypassed by exploiting instant apps. In this paper, we explore the weakness of the existing app link mechanism and propose three feasible hijacking attacks. Our findings show that even popular apps are subject to these attacks, such as Twitter, Whatsapp, Facebook Message. Our observation is confirmed by Google. To measure the severity of these vulnerabilities, we develop an automatic tool to detect vulnerable apps, and perform a large-scale empirical study on 400,000 Android apps. Experiment results suggest that app link hijacking vulnerabilities are prevalent in the ecosystem. Specifically, 27.1% apps are vulnerable to link hijacking with smart text selection (STS); 30.0% apps are vulnerable to link hijacking without STS, and all instant apps are vulnerable to instant app attack. We provide an in-depth understanding of the mechanisms behind these types of attacks. Furthermore, we propose the corresponding detection and defense methods that can successfully prevent the proposed hijackings for all the evaluated apps, thus raising the bar against the attacks on Android app links. Our insights and findings demonstrate the urgency to identify and prevent app link hijacking attacks.